Surfing Magnetic Waves in the Solar Atmosphere

Surfing Magnetic Waves in the Solar Atmosphere

How the Solar Wind Gets Up to Speed

July
8, 1999:Â The high-speed portion of the solar wind achieves
its unexpectedly high velocity -- up to 500 miles per second
-- by "surfing" magnetic waves in the Sun's outer atmosphere,
according to observations made by two spacecraft. For 37 years,
solar scientists have been puzzled by the fact that the high-speed
solar wind travels twice as fast as predicted by theory. Observations
and theoretical analyses have discovered a surprising explanation
for this mystery: magnetic waves.

The observations were made using instruments aboard NASA's
Spartan 201 spacecraft, deployed from the Space Shuttle during
the STS-95 mission, and the international Solar and Heliospheric
Observatory (SOHO). "The mystery was first presented by
the Mariner 2 spacecraft in 1962, the same year as Glenn's first
flight," said Dr. Marcia Neugebauer of NASA's Jet Propulsion
Laboratory, Pasadena, CA, the co-principal investigator of the
solar wind instrument on Mariner 2. "The new observations
made by SOHO and by the Spartan 201 mission during Glenn's return
to space put us much closer to finally unraveling the mystery
of the acceleration of the solar wind."

The outermost solar atmosphere, or corona, is an extremely
tenuous, electrically charged gas that is seen from Earth only
during a total eclipse of the Sun by the moon, when it appears
as a shimmering white veil surrounding the black lunar disk.
Using Spartan and SOHO, scientists have detected rapidly vibrating
magnetic fields within the corona that form magnetic waves that
appear to accelerate the solar wind. "These vibrating magnetic
waves give solar wind particles a push, just like an ocean wave
gives a surfer a ride," said Dr. John Kohl, a senior astrophysicist
at the Smithsonian Astrophysical Observatory in Cambridge, MA,
and the principal investigator for ultraviolet spectrometers
aboard SOHO and the Spartan 201.

The electrical charges of solar wind particles, or ions,
force them to spiral around invisible magnetic lines in the corona
as they rush into space. When the lines vibrate, as they do in
a magnetic wave, the spiraling ions are accelerated out and away
from the Sun. The presence of magnetic waves in the corona was
inferred by observing the motions of these solar wind ions. "We
were quite surprised to find that heavier oxygen ions actually
moved faster than lighter hydrogen ions," said Dr. Steven
Cranmer of the Harvard-Smithsonian Center for Astrophysics in
Cambridge, MA. "The best explanation for this is a magnetic
field line that wiggles back and forth in the same amount of
time that oxygen ions take to spiral around it. Just as a child
riding on a swing moves higher if someone pushes with the right
rhythm, the resonant magnetic waves give a boost to the oxygen
ions."

Above: This image shows three frames from a QuickTime
animation (click the image
to view) illustrating how magnetic waves in the Sun's outer atmosphere
(corna) accelerate the solar wind. The corona is seen as a feathery
yellow ring around the lunar disk during eclipse in the first
image, the particles making up the solar wind (red and green)
spiral around the magnetic field lines (white lines), accelerating
away from the Sun. The spiraling solar wind particles take energy
from the magnetic waves, canceling them out as the particles
rush into space.

The scientists believe there are magnetic waves in the
corona with many different "wiggling periods," or frequencies.
These waves accelerate various solar wind particles that spiral
around the field lines at different rates. The observations also
will help scientists better understand solar regions called coronal
holes. "Solar winds from these regions enhance energetic
electrons in the Earth's Van Allen radiation belts, which can
sometimes affect the electrical systems aboard Earth-orbiting
satellites," said Joseph W. Hirman, Chief of the Division
for Space Weather Operations at the Space Environment Center
operated by the National Oceanic and Atmospheric Administration
in Boulder, CO.

Even with this major discovery, there are questions left to
answer. "The observations have made it abundantly clear
that heavy particles like oxygen 'surf' on the waves, and there
is also mounting evidence that waves are responsible for accelerating
the hydrogen ions, the most common constituent of the solar wind,"
Cranmer said. "Other common particles, such as helium, have
never been observed in the accelerating part of the corona, and
new observations also are needed to refine our understanding
of how the waves interact with the solar wind as a whole."

The SOHO mission is sponsored by NASA and the European Space
Agency. This research was published in the June 20 edition of
the Astrophysical Journal.